Communication over mobile devices is known. Typically, the mobile communication device ("the mobile") has a direct RF or microwave communication link with a base station. The geographic area within which a mobile maintains the direct communication link with the base station is defined as a cell, a single base station thereby establishing a single cell. The plurality of base stations communicate with each other via cables and/or through existing telephone infrastructure. In order for a mobile to operate effectively in a large geographic area, there is a plurality of base stations with overlapping cells establishing an area of coverage over which a mobile may establish a communication link. When a mobile to base communication link is established, the base station further routes the transmission from the mobile to its intended destination. As the mobile travels throughout the geographic area, it passes in and out of established cells. The direct communication link between the mobile and the base station changes from a first base station to a second base station as the mobile moves out of a first cell and into a second cell. The transfer of the direct communication link from the first base station to the second base station is preferably seamless in that occurs without operator intervention and without loss of information during the transfer.
Various systems are known to effect the necessary transfer from the first base station to the second base station when the quality of the communication link is no longer acceptable. U.S. Pat. No. 4,308,429 discloses a method whereby a cell transfer or roaming decision is made centrally based on an assessment of the signal quality from the mobile to the base station. In the '429 patent disclosure, the first base station monitors a signal strength received by the mobile for the established communication link. If the received signal strength is below a certain threshold value, the first base station polls adjacent base stations to find a second base station having a stronger received signal strength from the mobile. The first base station then commands the mobile to transfer the direct communication link from the first base station to the second base station that has been identified as having the stronger received signal strength. In a voice communication system, signal strength is an acceptable parameter with which to assess transmission integrity. In a communication system that transmits data, however, the signal strength may not accurately indicate whether the established communication link is adequately communicating the informational content of the transmitted signal. Additionally, the centralized cell transfer initiation requires communication and arbitration between base stations. This inter-base station communication generates administrative traffic flow and complicates the system. The increased traffic flow represents system administration overhead which occupies communication bandwidth without contributing to communication capacity, thereby reducing the efficiency of the system. The centralized cell transfer initiation method also assumes that all base stations can identify all mobiles and can measure a mobile's transmission parameter in order to make a relative judgment as to which base station can better service a mobile at any one time. Due to certain practical limitations, the assumption that all base stations make similar measurements is not consistently true. As a result, a cell transfer may be initiated in error.
An alternative system for cell transfer is disclosed in U.S. Pat. No. 4,829,519 in which the mobile makes the decision to transfer the direct communication link to a different cell when the mobile detects a bit error rate above a certain exit threshold value. In the '519 patent disclosure, each base station in a plurality of base stations periodically transmits a known quality assessment message. The mobile continuously assesses the bit error rate in the known quality assessment message transmission and makes a decision to search for a new cell if the bit error rate is above a certain exit threshold value. The mobile then searches for a second base station by monitoring the bit error rate of the known quality assessment message from a plurality of base stations until the bit error rate of one of the base stations is below a certain entry threshold value defining a second base station. The mobile then transfers the direct communication link to the second base station. The cell transfer is initiated based upon an assessment of the forward channel communication. Disadvantageously, if the reverse channel is disadvantaged relative to the forward channel, the assessment of the forward channel quality is not a true measure of the quality of the communications link.
There is a need, therefore, for an improved cell transfer method that solves the disadvantages present in the prior art.